1. Field of the invention
The present invention relates to the method and apparatus for making magnetic recording media, such as perpendicular magnetic recording tapes, and, more particularly, to the method and apparatus suitable for the mass production of the magnetic recording media.
2. Description of the Prior Art
As shown in FIG. 1, a magnetic recording medium, such as a magnetic tape, generally comprises a substrate 2 made of a non-magnetic material and a magnetic layer 4 made of magnetic material deposited on substrate 2.
For high density recording, it is preferable to make layer 4 with a ferromagnetic thin film, such as Co, Ni, Fe and their alloys or oxides which have a merit in providing a high residual flux density.
In a case that the magnetic thin film has a tendency to be magnetized in a direction perpendicular to the face of the base (perpendicular magnetic anisotropy), still higher density recording can be achieved. Such a magnetic recording medium is referred to as a perpendicular magnetic recording medium, which is made of Co-Cr, Co-Cr-Rh, etc.
Generally, perpendicular magnetic recording media are produced by the method of sputtering, vacuum deposition, or ion plating, which are generally called a dry deposition process.
In the sputtering method, a magnetic film, e.g., a Co-Cr film is deposited by the step of sputtering on a base tape made of a non-magnetic material, such as PET (polyethylene terephthalate), polyamide, or polyimide plastic film. To proceed the sputtering process at a high speed, various sputtering methods are proposed, such as a Targets Facing Type, or a magnetron type, which are disclosed in detail in a Japanese Article "New sputtering methods for forming films" by M. Naoe which is introduced in a magazine published by the Institute of Applied Physics of Japan (Vol 48, No. 6).
In FIG. 2, an example of a prior art apparatus for making magnetic recording tapes through the Targets Facing Type of Sputtering system is shown. The apparatus has an air tight chamber 22 into which argon gas is introduced. A substrate, such as a plastic tape 6 is wound on supply reel 8 and take-up reel 10, and a tape portion extending between reels 8 and 10 is stretched around a can roller 12. When a high negative voltage from a source 20 is applied to targets 16a and 16b, each formed by, e.g., Co-Cr alloy plate, argon gas is ionized to produce a plasma discharge, which is confined between target 16a and target 16b by the magnets 18a and 18b coupled to targets 16a and 16b. When the positively charged ionized argon strikes on the target, particles of Co-Cr alloy eject from the target by sputtering and impinge on the tape surface where the tape portion is facing the targets.
The tape slowly moves in the direction indicated by an arrow, and a shield, or a can mask, 14 is provided closely adjacent can roller 12 at the upstream side of the tape movement at an angle .theta.i with respect to an axis X extending from the center of can roller 12 and the center of the targets. The larger the angle .theta.i, the more the amount of particles obliquely deposited from target 16b at the edge of the shield 14, which can be assumed to result in a great change of the film properties. Thus, the angle .theta.i is one of the important parameters which affect the properties of the deposited films. The angle .theta.i is referred to as an initial incident angle for convenience. Thereafter, no matter what the incident angle is, the following particles, deposited over the first particle, will build up a crystalline column of Co-Cr at the angle .theta.i with respect to the normal to the plane of the tape. Thus, in order to build up the crystalline column of Co-Cr as close as possible to the surface normal of the tape, it is preferable to move or widen the shield 14 to make the angle .theta.i as close as possible to zero.
However, if shield 14 is widened to make the angle .theta.i equal to or close to zero, the deposition effecting distance becomes short, requiring a slow speed advance of the tape to enable the growth of the Co-Cr crystalline columns to the required height, or otherwise, the Co-Cr crystalline layer, defined by the densely packed Co-Cr crystalline columns, will not have a required thickness.
Magnetic tapes manufactured by the method of sputtering with a small initial incident angle .theta.i is superior in the point that the deposited magnetic film has a high quality, but it takes disadvantage in that the deposition speed is still very slow, even when the above-mentioned Targets Facing Type or magnetron type is employed, particularly when it is compared with the sputtering with a wide initial incident angle .theta.i, or with other dry deposition processes, such as vacuum deposition or ion plating.
In FIG. 3, an example of a prior art apparatus for making magnetic recording tapes through the vacuum deposition system is shown, and is disclosed, e.g., in an article "Co-Cr Perpendicular Magnetic Recording Tape by Vacuum Deposition" by R. Sugita, given in IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp 687-692. The apparatus has an air tight chamber 22 in which a Co-Cr alloy ingot 23 contained in a plate 24 is provided. The ingot is heated by a suitable heating means, such as an electronic beam, so as to evaporate the Co-Cr alloy, The Co-Cr vapor is deposited on the substrate tape 6 stretched around can roller 12. The shields provided closely adjacent can roller 12 restrict the incident angle of the vaporized Co-Cr particle impinging on the substrate tape 6.
Magnetic tapes manufactured by the method of vacuum deposition is superior in the point that the deposition speed is fast, but the deposited magnetic film has a poor quality. Also, since the temperature of the substrate tape must be heated up to a quite high temperature, such as 300.degree. C., the substrate tapes which are widely used, e.g., PET tapes can not be used. Instead, it is necessary to use a heat-resistant tape, such as polyamide or polyimide tapes which are usually very expensive. The same can be said to the tape formed by the ion plating system.
Thus, the problem in the prior art method or apparatus for making the magnetic recording media, such as perpendicular magnetic recording tapes, is in the difficulty in making high quality magnetic recording media at high manufacturing speed.